1. Field of the Invention
The present invention relates to non-magnetic toner, an image forming apparatus and a process cartridge.
2. Description of the Related Art
Conventionally, as fixing systems of the toner, contact heating fixing systems such as heat roll fixing system are widely employed. A fixing apparatus employed for the heat roll fixing system comprises a heating roll and a pressurizing roll. A toner image is melted to fix on a recording sheet by passing the recording sheet bearing the toner image through a pressure welding section (nip section) between the heating roll and the pressurizing roll.
In the contact heating fixing system typified by the heat roll fixing system, the fixing is performed by contacting a surface of a heating member (e.g., heating roll) in the contact heating fixing apparatus with the toner image. Thus, it is necessary to prevent an offset phenomenon that a part of the toner image adheres to the heating member and transfers onto a subsequent recording sheet to stain the sheet.
For preventing the offset phenomenon, technology in which a fixing oil such as silicone oil is applied to or impregnated in the heating roll and the pressurizing roll in the fixing apparatus has been known. However, in terms of downsizing and reducing cost of the fixing apparatus, oilless fixing apparatuses omitting a fixing oil imparting mechanism or fixing apparatuses in which an amount of the fixing oil to be applied has been reduced are employed. When such a fixing apparatus is employed, a releasing agent as an anti-offset agent is added to the toner.
In the case of the heating fixing system, to save energy, it is more preferable that a heating temperature is lower as possible. However, in order to accomplish this, when a thermal property of a binding resin is designed to be too low, a heat resistant storage stability is deteriorated and blocking and the like occur. In order to balance them, it is advantageous to use a polyester resin for the binding resin. The polyester resin has lower viscosity and higher elasticity than vinyl based copolymer resins, and is excellent in fixing property at low temperature and good in heat resistant storage stability.
However, when the toner in which the sufficient amount of the releasing agent has been added for preventing the offset is produced by a conventional pulverization method, the releasing agent is abundantly exposed on the surface of the toner, resulting in the occurrence of filming and blocking. Meanwhile, when the toner is produced by so-called polymerization methods such as suspension polymerization (see Japanese Patent No. 3195362) in which a polymerizable monomer is polymerized in a water-based medium or emulsification polymerization (see Japanese Patent Application Laid-Open (JP-A) No. 2002-116574) in which fine particles are previously made by the emulsification polymerization and then aggregated, the sufficient amount of the releasing agent can be relatively enfolded, but the polymerization is performed in the water-based medium in the suspension polymerization and the emulsification polymerization. Thus, the vinyl based copolymer resins are generally used, and it is difficult to use the polyester resin polymerized at high temperature of about 200° C. or above.
As the method for granulating the toner using the polyester resin, a so-called dissolution suspension method in which the previously polymerized resin is dissolved in an organic solvent and granulated in the water-based medium is known. In this method, a molecular weight of the starting resin directly corresponds to the molecular weight of the toner. To control the thermal property of the toner, it is common to mix a low molecular weight resin and a high molecular weight resin to use. However, when the high molecular weight resin is added, the viscosity of the solution becomes too high, resulting in deterioration of granulability. Thus, the high molecular weight resin can not be used. Therefore, there is no other choice than making the molecular weight of the low molecular weight resin set higher, and this is disadvantageous to the fixing at low temperature.
Thus, there is the method in which modified polyester having a reactive functional group is used in place of the high molecular weight resin, and the molecular weight is controlled by performing an extending and/or crosslinking reaction after the granulation. When this method is used, it becomes possible to control the thermal property of the toner, but it is insufficient to control the toner structure, and a colorant and the releasing agent are easily exposed on the toner surface. When the colorant is exposed on the surface, a charging performance of the toner is changed. In particular, the narrower a particle diameter distribution is, the effect tends to become more remarkable. The releasing agent is exposed more abundantly on the surface, this is more advantageous to the fixing property, but problems occur in development durability, member staining such as filming and thermal stability. In particular, when the releasing agent having the low viscosity is used, the effect is remarkable, and for example oxidation affects dispersibility and charge property.
Meanwhile, when silicon oxide (SiO2) is used as a major ingredient of an externally adding agent, it is known that the charge property of the toner is changed by burying it in the toner surface.